Specialized filtration equipment is commonly mounted adjacent the ceiling of so-called "clean rooms" for the removal of substantially all of the particulate contamination in the air within the room. This equipment is frequently in the form of a group of modular units, each having its own blower taking in air from above the filters, and forcing it downward through the filters and into the room. Space above the modular units is often at a premium, resulting in efforts to reduce the overall height of the units.
This effort has encountered a problem. Air must be pressurized by the blower in a chamber above the filters, and flow somehow equalized over the entire filter. The centrifugal blowers commonly used are to a large extent immersed in the equalization chamber, and produce a high-order turbulence throughout the chamber. Somehow, these horizontal turbulence velocities must be converted to uniform vertical flow with a minimum of energy loss. This is especially difficult when the immersion of the blower into the chamber reduces the vertical space available for lateral flow.
Perforated plates interposed between the equalization chamber and the top of the filter have been used for redirecting the turbulence velocities, but the known forms of these have not produced a sufficiently uniform flow within desired limits of pressure drop. Standard HEPA filter units are usually about four feet long by two feet wide, and are pleated over the top surface in a direction parallel to the shorter dimension. Diffuser plates perforated with elongated openings arranged with the length of the openings parallel to the pleats have been used with only partial success. These appear in U.S. Pat. No. 5,014,608, issued on May 14, 1991. Another form of perforated diffuser plate has a dense array of uniformly spaced small circular holes. These also have proven to be only partially satisfactory.